Alkyne insertion into the M-P and M-H bonds (M=Pd, Ni, Pt, and Rh): A theoretical mechanistic study of the C-P and C-H bond-formation steps

In hydrogen-metal-phosphorus (H-M-P) transition metal complexes (proposed as intermediates of H-P bond addition to alkynes in the catalytic hydrophosphorylation, hydrophosphinylation, and hydrophospination reactions), alkyne insertion into the metal-hydrogen bond was found much more facile compared to alkyne insertion into the metal-phosphorus bond. The conclusion was verified for different metals (Pd, Ni, Pt, and Rh), ligands, and phosphorus groups at various theory levels (B3LYP, B3PW91, BLYP, MP2, and ONIOM). The relative reactivity of the metal complexes in the reaction with alkynes was estimated and decreased in the order of Ni>Pd>Rh>Pt. A trend in relative reactivity was established for various types of phosphorus groups: PR(2)>P(O)R(2)>P(O)(OR)(2), which showed a decrease in rate upon increasing the number of the oxygen atoms attached to the phosphorus center.